This invention relates to elastic substantially linear olefin polymers having improved processability, e.g., low susceptibilty to melt fracture, even under high shear stress conditions. Such substantially linear ethylene polymers have a critical shear rate at the onset of surface melt fracture substantially higher than, and a processing index substantially less than, that of a linear polyethylene at the same molecular weight distribution and melt index.
Molecular weight distribution (MWD), or polydispersity, is a well known variable in polymers. The molecular weight distribution, sometimes described as the ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) (i.e., Mw/Mn) can be measured directly, e.g., by gel permeation chromatography techniques, or more routinely, by measuring I10/I2 ratio, as described in ASTM D-1238. For linear polyolefins. especially linear polyethylene, it is well known that as Mw/Mn increases, I10/I2 also increases.
John Dealy in xe2x80x9cMelt Rheology and Its Role in Plastics Processingxe2x80x9d (Van Nostrand Reinhold, 1990) page 597 discloses that ASTM D-1238 is employed with different loads in order to obtain an estimate of the shear rate dependence of melt viscosity, which is sensitive to weight average molecular weight (Mw) and number average molecular weight (Mn).
Bersted in Journal of Applied Polymer Science Vol. 19, page 2167-2177 (1975) theorized the relationship between molecular weight distribution and steady shear melt viscosity for linear polymer systems. He also showed that the broader MWD material exhibits a higher shear rate or shear stress dependency.
Ramamurthy in Journal of Rheology, 30(2), 337 -357 (1986), and Moynihan, Baird and Ramanathan in Journal of Non-Newtonian Fluid Mechanics, 36, 255-263 (1990), both disclose that the onset of sharkskin (ie., melt fracture) for linear low density polyethylene (LLDPE) occurs at an apparent shear stress of 1-1.4xc3x97106 dyne/cm2, which was observed to be coincident with the change in slope of the flow curve. Ramamurthy also discloses that the onset of surface melt fracture or of gross melt fracture for high pressure low density polyethylene (HP-LDPE) occurs at an apparent shear stress of about 0.13 MPa (1.3xc3x97106 dynes/cm2).
Kalika and Denn in Journal of Rheology, 31, 815-834 (1987) confirmed the surface defects or sharkskin phenomena for LLDPE, but the results of their work determined a critical shear stress of 2.3xc3x97106 dyne/cm2, significantly higher than that found by Ramamurthy and Moynihan et al. International Patent Application (Publication No. WO 90/03414) published Apr. 5, 1990, discloses linear ethylene initerpolymer blends with narrow molecular weight distribution and narrow short chain branching distributions (SCBDs). The melt processibility of the interpolymer blends is controlled by blending different molecular weight interpolymers having different narrow molecular weight distributions and different SCBDs.
Exxon Chemical Company, in the Preprints of Polyolefins VII International Conference, page 45-66, Feb. 24-27 1991, disclose that the narrow molecular weight distribution (NMWD) resins produced by their EXXPOL(trademark) technology have higher melt viscosity and lower melt strength than conventional Ziegler resins at the same melt index. In a recent publication, Exxon Chemical Company has also taught that NMWD polymers made using a single site catalyst create the potential for melt fracture (xe2x80x9cNew Specialty Linear Polymers (SLP) For Power Cables,xe2x80x9d by Monica Hendewerk and Lawrence Spenadel, presented at IEEE meeting in Dallas, Tex., September, 1991).
Previously known narrow molecular weight distribution linear polymers disadvantageously possessed low shear sensitivity or low I10/I2 value, which limits the extrudability of such polymers. Additionally, such polymers possessed low melt elasticity, causing problems in melt fabrication such as film forming processes or blow molding processes (e.g., sustaining a bubble in the blown film process, or sag in the blow molding process etc.). Finally, such resins also experienced melt fracture surface properties at relatively low extrusion rates thereby processing unacceptably.
Elastic substantially linear olefin polymers have now been discovered which have unusual properties, including an unusual combination of properties, which leads to enhanced processabilily of the novel polymers. The substantially linear olefin polymers have the process ability similar to highly branched low density polyethylene, but the strength in toughness of linear low density polyethylene. The substantially linear olefin polymer are characterized as having a critical shear rate at onset of surface melt fracture of at least 50 percent greater than the critical shear rate at the onset of surface melt fracture of a linear olefin polymer having about the same I2 and Mw/Mn.
The elastic substantially linear olefin polymers also have a processing index (PI) less than or equal to about 70% of the PI of a comparative linear olefin polymer at about the same I2 and Mw/Mn.
The elastic substantially linear olefin polymers also have a melt flow ratio, I10/I2, xe2x89xa75.63, and a molecular weight distribution, Mw/Mn, defined by the equation:
Mw/Mnxe2x89xa6(I10/I2)xe2x88x924.63.
Compositions comprising the substantially linear olefin polymer and at least one other natural or synthetic polymer are also within the scope of the invention.
Elastic substantially linear olefin polymers comprising ethylene homopolymers or an interpolymer of ethylene with at least one C3-C20 xcex1-olefin copolymers are especially preferred.